How to install exterior foam insulation where new addition meets old house
I am adding an addition to my house in New York this summer. The current house is basically square with a large porch across the entire front. The addition is going to be added as a rectangle that will create a transverse roof. The addition will extend beyond the front of the old house by 4 feet and above the roof by a foot or so.
My plan was to insulate by using open cell spray foam on the interior of 2×6 walls and 4″ of rigid foam on the exterior. My question is how to deal with the section of the addition that adjoins the old house. If I add the rigid foam to the exterior of that wall it will throw off the balance of the house and porch. Everything will be off center. Looking for how to deal with where the two sections of house meet and insulating them.
Additionally, I have been very confused with how to insulate the vaulted roof. I am not sure if I should go with open or closed cell foam or some other insulation with rigid foam on the exterior? After reading several articles on this site, there seems to be much contradiction concerning moisture management.
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Where in New York? Got a ZIP code?
Location matters- NY state ranges from climate zone 4A to climate zone 7A, and it makes a huge difference in what's needed for the vaulted ceiling.
I can't quite figure out what the question is about the walls. A sketch might make it clearer.
I'm right on the border of zone 5 and 6. Picture attached. Hopefully that helps.
Caine,
It's true that adding exterior insulation to the house (as drawn) affects the symmetry. There are two possible solutions:
1. If we're talking about a total of 6 inches, simply plan ahead and locate the stud walls of the new addition 6 inches farther away from the existing house, to preserve the symmetry you value.
2. Don't worry so much about symmetry.
If you want to insulate the sloped roof assemblies of your addition, and you are trying to choose between (a) open-cell spray foam under the roof sheathing, (b) closed-cell spray foam under the roof sheathing, or (c) rigid foam above the roof sheathing, then I would certainly choose (c), since it's the only option that addresses thermal bridging through the rafters. For more information, see How to Install Rigid Foam On Top of Roof Sheathing,
In zone 5 an unvented roof needs a minimum of 40% of the total R to be low-permeance on the exterior side of the higher permeance insulation (open cell foam or fiber insulation) to limit interior moisture drives from affecting the roof deck, and for dew point control at the high perm/low perm boundary.
In zone 6 it takes a minimum of 50%.
So at a code minimum R49 you would need R20 of closed cell foam (or rigid foam above the roof deck) in zone 5, R25 in zone 6. If you go higher R than that, the low-perm layer has to grow proportionally.
If you have access to reclaimed roofing foam (it's dirt cheap, cheaper than batts, with multiple vendors within driving distance of the Hudson valley), installing 6" of rigid foam on the exterior and 5.5" of open cell foam in 2x6 rafters under the roof deck gets you there. Even with virgin-stock polyiso, 6" exterior foam would run about $4 per square foot and deliver a continuous R30-ish performance. For about the same money 4" of closed cell foam under the roof deck would only deliver ~R24 at center cavity, and would require another R25 of something below that, but thermal bridging of the rafters would cut into performance considerably.
A 2 x12 rafter bay filled with 0.7lb density open cell foam gets you R46- almost to code min, and would be fine if you use a continuous sheet of 2-mil nylon (Certainteed MemBrain) under the gypsum board. Half pound foam would be safe with the 2-mil nylon too but would be only R42-R43. If doing it with foam+ fiber, 2" of closed cell foam on the roof deck is sufficient protection for the roof deck, and the 2-mil nylon would have to be detailed as an air barrier.